This section is intended to provide a background or context to the invention disclosed below. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived, implemented, or described. Therefore, unless otherwise explicitly indicated herein, what is described in this section is not prior art to the description in this application and is not admitted to be prior art by inclusion in this section.
Microfluidic-based devices offer benefits as they are in effect miniaturized laboratories with advantages of low-energy, small sample, and bio-receptor volume consumption; high integration, multiplexing, and compactness; fast results; and low cost. Moreover, such devices have the potential for applications as research platforms as well as point-of-care devices.
Microfluidics facilitate touchless manipulation of single cells, organisms, or particles through the exploitation of the “dielectrophoretic” effect. A dielectrophoretic (DEP) force arises from the polarization of otherwise electrically neutral particles or cells when suspended in a non-homogeneous electric field. This requires creating an electric field gradient within the sample fluid, often done with an arrangement of planar metallic electrodes integrated in the microfluidic channel in contact with the fluid.
This polarization occurs due to the imbalanced distribution of bonded charges induced by the electric field and acts to attract/repel cells to/from electric field maxima for positive/negative dielectrophoretic force, as described in the following equation:FDEP(r;V)=2πεmR3 Re[CM]∇|E(r;V)|2  (1),where:                R=radius (particle size);        CM=(ε*p−ε*m)/(ε*p+2ε*m)=Claussius-Mossotti Factor (complex coefficient arising from the relation between the complex electrical properties of the particle, ε*p, and the suspending medium, ε*m); and        |E (r; V)|2=spatial distribution and excitation configuration of the electric field intensity as a function of the electrode design as well as the applied voltage V.        
The complex permittivity (an electrical or material property) of either the medium, ε*m, or the particle, ε*p, is given by ε*=ε−jσ/ω, with ε=ε0εr being the absolute permittivity of the material, εr being the relative permittivity, ε0 the free space permittivity, σ the conductivity, and ω the angular frequency. The imaginary unit j is defined as j2=−1. These forces depend not only on the geometrical configuration and excitation scheme of the electric field but also on the electrical properties of the cell or particle and of its suspending medium as well as its size; hence, can be used for particle discrimination, separation, isolation, or concentration, being useful for sample processing.
Integrated electrodes in microfluidic channels can be used with other purposes in addition to generating DEP forces (both positive and negative) such as electrical sensing (impedance, capacitance, etc.), optical illumination and detection, heating mechanism to induce reactions, etc.
Microfluidic devices often require transporting fluids through the micro-channels, usually done by coupling to external pumping peripherals with fluid tubes and connectors. Such flow-based microfluidic devices can be made more portable and autonomous by integrating a capillary pump onto the device substrate capable of sustaining a flow rate in the channel Such capillary pump refers to a widening structure within the micro channel filled with an array of pillars capable of pulling fluid along the channel by means of capillary pressure. The flow rate and volume of fluid this structure can remove depends strongly on its geometry, width, size, and placement of the pillars; hence, flow speed can vary from chip design to chip design, and potentially from chip to chip, due to manufacturing variability or temperature changes causing increased evaporation. In addition, some flow speed variability can be observed at the various stages of capillary pump filling.
To manipulate particles in a microfluidic channel with integrated electrodes, such electrodes need to be powered at a certain voltage, where the optimum voltage to be applied to the electrodes is highly dependent on the velocity of the particles being transported within the flow. Means for measuring flow velocity exist; however, they require expensive equipment and can only be performed in a laboratory environment. On the other hand, simpler means of measuring flow or particle speed at the time of an experiment are needed in order to readjust the voltage settings to accommodate variations in flow speed.
The current invention moves beyond the current techniques and/or materials.
Abbreviations that may be found in the specification and/or the drawing figures are defined in the text where appropriate.